(a) Field of the Invention
The present invention relates to a method for focusing an objective lens of endoscope.
(b) Description of the Prior Art
It is required for an endoscope to be capable of observing areas as wide as possible in case of observing an object located at a long distance and also to observe a narrow area with said endoscope kept close to said area so as to comprehend abnormal portion minutely without overlooking. For observing an object located at a short distance, an endoscope should preferably have high magnification and must permit proper focusing at such high magnification level.
For focusing an objective lens of an endoscope, there has conventionally been known a method to displace an optical fiber bundle 1 as shown in FIG. 1A and FIG. 1B (for example, the method disclosed by Japanese Published Unexamined Utility Model Application No. 99087/74). Speaking more concretely, the optical fiber bundle 1 is retreated for a distance of .DELTA..sub.0 =f.sub.0 .beta..sub.0 (wherein the reference symbol f.sub.0 represents focal length of an objective lens 2 and the reference symbol .beta..sub.0 designates magnification level of the objective lens 2) for setting the endoscope at the position to observe an object located at a short distance shown in FIG. 1B from the position for observing an object located at a long distance shown in FIG. 1A. As is clear from the figures, this method had a drawback that the principal rays falls on the optical fiber bundle at a large angle of incidence and transmitted light intensity is lower at marginal portions of the optical fiber bundle, thereby darkening marginal portions on an image formed with the optical fiber bundle.
Further, there has conventionally been known another method for focusing an objective lens of an endoscope which comprises dividing an objective lens 2' into two groups 2'a and 2'b as illustrated in FIG. 2A and FIG. 2B, and changing the distance between said two lens groups (for example, the method disclosed by Japanese Published Examined Utility Model Application No. 2194/75). Speaking more concretely, the lens group 2'b arranged on the side of the optical fiber bundle 1 is displaced for setting the endoscope at the position for observing an object O located at a short distance shown in FIG. 2B from the position at which said objective lens is focused on an object located at a long distance shown in FIG. 2A. However, this method also had a drawback that it cannot provide sufficiently high magnification level since the objective lens has a short focal length when it is focused with the object O located at a short distance.